This invention relates to iron-based, metallurgical powder compositions, and more particularly, to powder compositions that include alloying elements in particulate or powder form for enhancing the strength characteristics of resultant compacted parts.
Iron-based particles have long been used as a base material in the manufacture of structural components by powder metallurgical methods. The iron-based particles are first molded in a die under high pressures to produce the desired shape. After the molding step, the compacted or xe2x80x9cgreenxe2x80x9d component usually undergoes a sintering step to impart the necessary strength to the component.
The strength of the compacted and sintered component is greatly increased by the addition of certain alloying elements, usually in powder form, to the iron-based powder. Commonly used powder metallurgical compositions contain such alloying elements as carbon (in the form of graphite), nickel, copper, manganese, molybdenum, and chromium, among others. The level of these alloying elements can be as high as about 4-5 percent by weight of the powder composition. At the levels used, the cost associated with these alloying element additions can add up to a significant portion of the overall cost of the powder composition. Accordingly, it has always been of interest in the powder metallurgical industry to try to develop less costly alloying elements or compounds to reduce and/or replace entirely the commonly used alloying elements.
Furthermore, although highly useful, some of these alloying elements have undesired properties as well. For example, certain parts manufacturers desire to limit the amount of copper and/or nickel used in the powder metallurgy compositions that are used to form compacted parts due to the environmental and/or recycling regulations that regulate the use or disposal of those parts. The use of graphite is sometimes disadvantageous because it easily dusts out of the powder composition, leading to reduced performance of the compacted part due to the absence of the required amount of carbon for the powder mix.
The inclusion of alloying elements into the powder composition may either enhance or diminish the final part""s ductility, that is, the ability of the part to retain its shape after a strain is applied and removed. Certain parts applications require relatively good ductility properties for the final parts. Copper and nickel-containing powder metallurgy parts have low ductility and thus pose certain design constraints. Typically, the range of ductility for such parts is between 1.5 and 2 percent per inch. In certain applications, however, it is desirable for a powder metallurgy part to have ductilities in excess of 3 percent per inch.
As reported in the text Ferrous Powder Metallurgy, (1995), attempts have been made in the past, particularly work conducted by A. N. Klein et al., to use silicon as an alloying element to replace such alloying elements as copper, nickel, and molybdenum. The silicon was added to the iron powder in the elemental form, in the form of ferroalloys, or in special ternary FeSiMn master alloy formed by silicides. The use of silicon was found, however, to lead to excessive shrinkage of binary Fe-Si compacts in the range of usual compositions and compaction/sintering conditions. Elemental silicon powder typically has a silicon dioxide rich surface that is difficult to reduce back to silicon in sintering environment commonly used in the manufacture of powder metal parts. In addition, ferroalloys containing silicon are not compressible during molding and thus produce parts having inadequate sintered densities.
There exits a current and long felt need in the powder metallurgical industry to develop alternatives to the use of, or decrease the amount of, various common alloying elements in the powder mixes, such as copper and nickel. Any suitable alternative should be easily blended with the iron-based powder, and improve the strength and/or ductility characteristics of the compacted parts without significantly deteriorating various other powder or compacted part properties.
The present invention provides metallurgical powder compositions comprising as a major component a powder metallurgy base metal powder, such as iron-based and/or nickel-based powders, to which is blended a silicon carbide-containing powder. The silicon carbide-containing powder has been found to surprisingly enhance the strength and ductility of the final, sintered, compacted parts made from the metallurgical powder compositions. The properties of the final part have been found to be significantly improved if the xe2x80x9cgreenxe2x80x9d compacted part is sintered at temperatures above about 2150xc2x0 F., preferably above about 2200xc2x0 F., more preferably above about 2250xc2x0 F., and even more preferably above about 2300xc2x0 F.
The metallurgical powder compositions generally contain at least about 85 percent by weight of a powder metallurgy base metal powder such as an iron-based powder or a nickel-based powder. A silicon carbide-containing powder is also present in the metallurgical powder compositions in an amount to provide from about 0.05 to about 7.5 percent by weight silicon carbide. Preferably, the base metal powder is an iron-based powder or combination of such powders having a particle size distribution commonly used in the powder metallurgical industry. The silicon carbide is preferably blended into the composition as a silicon carbide powder that is at least about 95 percent pure silicon carbide. However, the silicon carbide-containing powder may be a binary, tertiary, etc. alloy of the silicon carbide with other powders used in metallurgical powder compositions. Alternatively, the silicon carbide-containing powder can be bonded, e.g., diffusion bonded, to the base metal powder, e.g., iron-based powder. The silicon carbide powder preferably has a particle size distribution such that it has a d50 value of below about 75 or 50 microns as determined by laser light scattering techniques, and may be angular, rectangular, needle-shaped, spherical, or any other shape.
The metallurgical powder compositions can optionally also contain any of the various other additives commonly used in such compositions. For example, the compositions can contain lubricants, binding agents, and other alloying elements or powders such as copper, nickel, manganese, and graphite.
The present invention also provides methods for the preparation of these metallurgical powder compositions and also methods for forming compacted and sintered metal parts from such compositions.